The present invention relates to an optical amplifier or laser oscillator used for, for example, 1.4-1.52 xcexcm-band optical communication.
Hitherto, 1.55-xcexcm band, which is the lowest loss wavelength of a silica fiber, and 1.3 xcexcm-band, which is zero dispersion wavelength, have been used in optical communication. Research and development of optical amplifiers with respect to these wavelength bands have been active, and considerable successes were achieved particularly for 1.55-xcexcm band. In recent years, there have been demands for Internet and data transfer to have a high-speed and a large-volume. Thus, wavelength division multiplexing (WDM) is put into practical use from the viewpoint of increasing communication volume. The communication volume may, however, become insufficient with the use of only 1.55 xcexcm-band. Thus, there were in recent years proposals for using other bands. In particular, there are strong demands for using the entire region of a wide transmitted wavelength range of quartz fiber of from 1.3 to 1.7 xcexcm. Thus, active research is going on for 1.47 xcexcm-band and 1.65 xcexcm-band Tm doped fiber amplifier.
There is a proposal for using a low phonon glass fiber doped with Tm for a 1.47 xcexcm-band amplification or laser oscillation (see xe2x80x9cRare-earth-doped fiber lasers and amplifiersxe2x80x9d by D. N. Payne et al., Proc. 14th Europ. Conf. Opt. Comm., Brighton, U.K. 1988, pages 49-53; and IEEE. J. Quantum Electronics Vol. 24(6) (1988) pages 920-923). The Tm energy level is shown in FIG. 1 of the present specification. The 1.47 xcexcm-band amplification is conducted by using a stimulated emission process of 3H4xe2x86x923F4. This actually makes it possible to have a wide gain and an amplification band range of from 1.4 to 1.52 xcexcm. Japanese Patent Unexamined Publication JP-A-5-275792 discloses a fiber laser and a fiber amplifier using a fluoride fiber having a core doped with Tm, and 1.06 xcexcm-band light as a pumping light. U.S. Pat. No. 5,366,937, corresponding to JP-A-4-265251, discloses a method for exciting 3H4 of Tm using a pump light wavelength of 0.7-0.89 xcexcm. JP-A-4-180279 discloses a two-steps excitation at 1.2 xcexcm band. It is, however, known to be difficult to obtain population inversion necessary for amplification or laser oscillation, since fluorescence lifetime of 3H4 level of Tm is shorter than that of 3F4. Such characteristic is referred to as xe2x80x9cself-terminating mechanismxe2x80x9d, and this makes it unsuitable for a highly efficient amplification or laser oscillation. Thus, there are several proposals for adding co-dopants (Eu, Tb, Ho and Pr and the like) in order to make the 3F4 lifetime shorter (see JP-A-4-265251; U.S. Pat. No. 5,341,237 corresponding to JP-A-5-145,168; and JP-A-7-45899). However, when these co-dopants are added in a large amount, the tail of the absorption band overlaps with the target wavelength range of 1.4-1.52 xcexcm-band. This may cause an unfavorable energy transfer and an adverse effect on the amplification efficiency of 1.4-1.52 xcexcm-band. Therefore, there is a limitation on the co-dopant concentration and amplification efficiency.
JP-A-2000-307176 discloses an optical fiber amplifier having a first pumping source for a pumping light in a wavelength range of 0.98-1.08 xcexcm and a second pumping source for a pumping light in a wavelength range of 1.08-1.25 xcexcm, 1.51-2.00 xcexcm or 0.77-0.84 xcexcm.
Aozasa et al., OFC2001 Technical Digest Series, PD1 discloses a high Tm3+ concentration doped fluoride fiber amplifier with one color 1.4 xcexcm pumping. Roy et al., OFC2001 Technical Digest Series, PD2 discloses the use a dual wavelength pumping at 1.24 xcexcm and 1.40 xcexcm for a thulium-doped fiber amplifier. Kasamatsu et al., Proceedings OAA ""99, PDP1 discloses a 1.50-xcexcm band gain-shifted thulium-doped fiber amplifier by using a dual wavelength pumping of 1.05 xcexcm and 1.56 xcexcm. European Patent Application EP 1039593 A2, corresponding to JP-A-2000-340865, also discloses a thulium-doped laser amplifier by using a dual wavelength pumping of 1.04-1.07 xcexcm and 1.53-1.90 xcexcm.
Low phonon energy glass materials are well known as up-conversion laser materials. Thus, when it is tried to amplify a signal light by using a low phonon energy glass material, the pumping energy is consumed by emission of shorter wavelength like a red or blue light. Therefore, 1.4-1.52 xcexcm-band emission efficiency is generally known to be extremely low. 1.4-1.52 xcexcm-band emission from 3H4 to 3F4 of Tm3+ is well known as a typical example of self-terminating system, because the fluorescence lifetime of 3H4 is shorter than that of 3F4. Thus, it is difficult to achieve a highly efficient laser oscillation itself (see J. Y. Allain et al., Electronics Letters vol. 25 (1989) pages 1660-1662). There are proposals for making the lifetime of 3F4 shorter by adding a co-dopant such as Eu, Tb, Ho, and Pr. These co-dopants also have a wide absorption wavelength band range. Therefore, when such co-dopant is added in an amount sufficient for making the 3F4 lifetime shorter, absorption may occur at 1.4-1.52 xcexcm-band. Furthermore, the emission efficiency at 1.4-1.52 xcexcm-band may be lowered by the up-conversion process through energy transfer.
It is possible to try to shorten lifetime of 3F4 by using an oscillation of 3F4xe2x86x923H6. This is, however, not preferable in obtainment of a 1.4-1.52 xcexcm-band optical amplifier or laser oscillator of a high efficiency, since nearly a half of the pumping light energy is lost.
It is therefore an object of the present invention to provide a 1.4-1.52 xcexcm-band optical amplifier or laser oscillator of a high efficiency.
According to the present invention, there is provided a 1.4-1.52 xcexcm-band optical amplifier comprising:
(a) a first pumping source for a first pumping light in a wavelength range defined as being not less than 0.65 xcexcm and being less than 0.77 xcexcm;
(b) a second pumping source for a second pumping light in a wavelength range of from 1.0 to 1.2 xcexcm;
(c) an optical multiplexer for combining said first and second pumping lights with a signal light, to produce a combined light; and
(d) a light amplifying optical waveguide for amplifying said signal light by receiving said combined light, said optical waveguide comprising a core doped with Tm.